time2posix, posix2time  convert seconds since the Epoch
Standard C Library (libc, lc)
#include <time.h>
time_t
time2posix(const time_t *t);
time_t
posix2time(const time_t *t);
IEEE Std 1003.11988 (``POSIX.1'') legislates that a time_t value of
536457599 shall correspond to "Wed Dec 31 23:59:59 GMT 1986." This
effectively implies that POSIX time_t's cannot include leap seconds and,
therefore, that the system time must be adjusted as each leap occurs.
If the time package is configured with leapsecond support enabled, however,
no such adjustment is needed and time_t values continue to increase
over leap events (as a true `seconds since...' value). This means that
these values will differ from those required by POSIX by the net number
of leap seconds inserted since the Epoch.
Typically this is not a problem as the type time_t is intended to be
(mostly) opaquetime_t values should only be obtainedfrom and passedto
functions such as time(3), localtime(3), mktime(3) and difftime(3). However,
IEEE Std 1003.11988 (``POSIX.1'') gives an arithmetic expression
for directly computing a time_t value from a given date/time, and the
same relationship is assumed by some (usually older) applications. Any
programs creating/dissecting time_t's using such a relationship will typically
not handle intervals over leap seconds correctly.
The time2posix() and posix2time() functions are provided to address this
time_t mismatch by converting between local time_t values and their POSIX
equivalents. This is done by accounting for the number of timebase
changes that would have taken place on a POSIX system as leap seconds
were inserted or deleted. These converted values can then be used in
lieu of correcting the older applications, or when communicating with
POSIXcompliant systems.
The time2posix() function is singlevalued. That is, every local time_t
corresponds to a single POSIX time_t. The posix2time() function is less
wellbehaved: for a positive leap second hit the result is not unique,
and for a negative leap second hit the corresponding POSIX time_t doesn't
exist so an adjacent value is returned. Both of these are good indicators
of the inferiority of the POSIX representation.
The following table summarizes the relationship between time_t and its
conversion to, and back from, the POSIX representation over the leap second
inserted at the end of June, 1993.
DATE TIME T X=time2posix(T) posix2time(X)
93/06/30 23:59:59 A+0 B+0 A+0
93/06/30 23:59:60 A+1 B+1 A+1 or A+2
93/07/01 00:00:00 A+2 B+1 A+1 or A+2
93/07/01 00:00:01 A+3 B+2 A+3
A leap second deletion would look like...
DATE TIME T X=time2posix(T) posix2time(X)
??/06/30 23:59:58 A+0 B+0 A+0
??/07/01 00:00:00 A+1 B+2 A+1
??/07/01 00:00:01 A+2 B+3 A+2
[Note: posix2time(B+1) => A+0 or A+1]
If leapsecond support is not enabled, local time_t's and POSIX time_t's
are equivalent, and both time2posix() and posix2time() degenerate to the
identity function.
difftime(3), localtime(3), mktime(3), time(3)
FreeBSD 5.2.1 May 1, 1996 FreeBSD 5.2.1 [ Back ]
